The present invention relates to a protective circuit for a printer driver.
In a serial printer in which a stepping motor is used for driving a print head for the purpose of spacing or line feed, an over-driving scheme is often employed for quickly moving the print head. In an over-driving scheme, a power supply having a voltage higher than the rated voltage of the motor is applied so that the current through the coil rises quickly. The application of the voltage is commenced when the coil of each phase begins to be selected, and terminated before the current through the coil becomes excessive. When the transistor is kept ON longer than usual, the current through the coil becomes excessive and the coil in the motor may burn out. To protect the motor coil from such an excessive current, a fuse is provided so that when the current becomes excessive, the fuse is blown and the current to the motor coil is interrupted.
A drawback in the above-mentioned prior art protective circuit is that it is necessary to exchange the fuse each time the protective circuit has operated to blow the fuse. This is a serious disadvantage particularly where the reason for the excessive current is a malfunction of the CPU in the control circuit, rather than the breakdown of the motor drive transistor.